Real-time, three-dimensional imaging has become an important challenge in the design of modern electronic image sensors. Many applications, such as robotics, biometrics, automobile security and navigation, medical imaging and surveillance demand a three-dimensional representation of the environment to avoid time-consuming processing steps that typically are used in ultra-sonic or radar imaging techniques. Optical systems can allow for very fast three-dimensional data acquisition, an (eye)-safe system set-up and high lateral resolution.
Time-of-flight techniques, for example, can facilitate fast optical acquisition of distance information. In TOF systems, the time that light needs to travel from the measurement system to the scene and back again corresponds directly to the distance R. The time-of-flight (TOF) may be determined by:TOF=2R/c, where c is the velocity of light. Sensors for real-time three-dimensional imaging systems typically include pixel-matrices composed of “smart” pixels that are capable of simultaneously delivering distance information for respective points in an imaged scene. In some TOF systems, a modulated light wave is emitted toward a scene, and the phase delay between the original and received light signals is used to extract the time or distance information. A special group of “smart” pixels, called demodulation or lock-in pixels, can be used to demodulate the optical wave impinging on the sensor.
Demodulation contrast (cdemod) is an important parameter for TOF pixels and defines the quality of the pixel's inherent demodulation efficiency. The demodulation contrast can be defined by the ratio between the amplitude (A) and offset (B) values when no background light is present, where the offset (B) corresponds to the mean number of electrons Asig generated exclusively by the signal component. Thus:
      c    demod    =                    A        B            ⁢              |                                                            without                ⁢                                                                  ⁢                back                ⁢                                  -                                                                                                        ground                ⁢                                                                  ⁢                lignt                                                          =          A              A        sig            The theoretical maximum for the demodulation contrast depends on the specific nature of the sampling process. The shorter the integration period, the higher the demodulation contrast can be. For example, in 2-tap pixels, in which each demodulation pixel samples and stores two samples in half the modulation period, the demodulation contrast (assuming sinusoidal light pulses) cannot exceed about 63%. In practice, various parasitic effects tend to reduce the actual demodulation contrast even further.